Robert Mugabe shows off his stained finger after casting his vote in Harare. Mugabe has been in power for 33 years. Photograph: Xinhua /Landov / Barcroft Media

President Robert Mugabe's party has claimed victory in Zimbabwe's elections by a landslide, while his main rival dismissed it as a "huge farce" and warned that an illegitimate result would plunge the country into a serious crisis.

Final results are not yet published but Zanu-PF said on Thursday it was confident of a resounding victory and state television reported wide gains for the party. By contrast, the Movement for Democratic Change (MDC), which had previously expressed confidence, appeared despondent and attacked the electoral process.

Saviour Kasukuwere, Zanu-PF's youth and indigenisation minister, predicted: "It's a landslide, a total annihilation of the MDC. Their project has completely failed. President Mugabe has won. The villagers in the countryside know the result.

"I'm excited. I was young at independence in 1980. Today I feel very independent. Our country is now firmly in our hands. It's a second coming."

Asked if Mugabe, who at 89 is Africa's oldest leader, would serve a full term in office, Kasukuwere replied: "We have elected President Mugabe to serve a full term."

Until Wednesday, the MDC had insisted that, despite the claims of vote rigging and intimidation, its supporters' weight in numbers would be enough to end Mugabe's 33-year rule. But the mood changed as the arithmetic began to sink in.

"This election has been a huge farce," said presidential candidate Morgan Tsvangirai, sombre in flat cap and jacket as he read a statement at MDC headquarters. "In our view, that election is null and void … It is a sham election that does not reflect the will of the people."

"For the above reasons, the election has been heavily manipulated," the prime minister said. "In our view, the outcome of this election is illegitimate. But more importantly, the shoddy manner in which it has been conducted and the consequent illegitimacy of the result will plunge this country into a serious crisis."

Tsvangirai's concerns were echoed by a foreign diplomat who said: "It is a very sad day for Zimbabwe. What we've seen is huge evidence of voters' roll irregularities, extra polling booths set up and voters bussed in, and that had a big impact. We have very grave concerns about the credibility of this election. It looks like a big steal."

The head of the independent Zimbabwe election support network, Solomon Zwana, said its observers found a wide range of problems and up to a million voters had been disenfranchised. It claimed that registration was higher and the turning away of voters lower in areas known to be Zanu-PF strongholds.

But a southern African electoral commissions team, which brought 25 observers, said there had been no reports of violence or intimidation and praised organisers for their transparency. "We believe these elections have been conducted in a credible and fair manner," it said. The African Union is yet to deliver its verdict.

Kasukuwere rejected Tsvangirai's claims as "absolute nonsense", demanding: "How can he talk of vote rigging when there are some MDC candidates who have won and some key members of ours who have lost. Would we rig against ourselves? He knows his party is divided. What's so special about losing an election?"

On Thursday afternoon, the MDC headquarters was shuttered and the party website was still advertising a pre-election rally and a "countdown to July 31".

Defeat would be a devastating blow to the 14-year-old party and raise questions over the future of Tsvangirai, who was making his third attempt to unseat Mugabe. Previous elections were marred by allegations of vote rigging, killings and torture; in 2008 Tsvangirai withdrew, citing violence against his supporters which left more than 200 people dead. Tsvangirai declined to take questions on how the MDC would react to the official result.

On Wednesday, Tendai Biti, the MDC secretary general, brushed off a query as to whether the MDC would protest in the courts or the streets, insisting that it was heading for victory. The MDC's final election rally in Harare drew an estimated record 100,000 people.

McDonald Lewanika, co-ordinator of the civil society group Crisis in Zimbabwe Coalition, called for people to challenge the result by any means necessary. He said: "This is no longer a legal issue; the MDC has gone to court on several occasions. The MDC must take this politically by rejecting the election and taking action to show Zanu-PF they have the people.

"They have to leave their laptops, leave their Range Rovers, leave their comfort zones and show they have got the people power. If people were to walk to State House or stop the announcement of the vote, let it be. They've got no choice because otherwise this is the end of it all. My vote has been stolen."

David Coltart, the education, sport, arts and culture minister, described it as a catastrophic day for the MDC, admitting: "There's no point putting a gloss on it. Zanu-PF will win a majority of seats in parliament and Robert Mugabe will get more than 50%."

Coltart, a member of the breakaway MDC faction led by Welshman Ncube, said he appeared to have lost his parliamentary seat, by just 19 votes to Thabitha Khumalo from Tsvangirai's MDC. Zanu-PF also did much better than expected.

He said there were "glaring anomalies" in his constituency, Bulawayo East. "The number of people voting for Mugabe and Zanu-PF bears no relation to historical trends and there are incredibly unusual voting patterns around military barracks," he said. "They created seven polling stations in a 2km radius of military barracks and I lost the election in those seven stations. In other stations, several hundred people were turned away, which also cost me. I seriously question the veracity of the result."

The discrepancies were mirrored across the country, he claimed, adding: "Zimbabwe has been subjected to electoral fraud on a massive scale."

Asked how the MDC factions should respond, Coltart said: "The problem of demonstrating in the streets is that you play into Zanu-PF's hands. They are a violent party. The supreme court is unreliable so we have extremely limited options. I tend to think it will be a combination of economic and international pressure: the great tragedy for this country is the economic crisis will be exacerbated by Zanu-PF."